The indirect taxes consisted--apart from the subordinate moneys
levied from roads, bridges, and canals--mainly of customs-duties.
The customs-duties of antiquity were, if not exclusively, at any
rate principally port-dues, less frequently frontier-dues, on
imports and exports destined for sale, and were levied by each
community in its ports and its territory at discretion.

The Romans
recognized this principle generally, in so far as their original
customs-domain did not extend farther than the range of the Roman
franchise and the limit of the customs was by no means coincident
with the limits of the empire, so that a general imperial tariff
was unknown: it was only by means of state-treaty that a total
exemption from customs-dues in the client communities was secured
for the Roman state, and in various cases at least favourable
term for the Roman burgess.

But in those districts, which had
not been admitted to alliance with Rome but were in the condition
of subjects proper and had not acquired immunity, the customs fell
as a matter of course to the proper sovereign, that is, to the Roman
community; and in consequence of this several larger regions within
the empire were constituted as separate Roman customs-districts, in
which the several communities allied or privileged with immunity
were marked off as exempt from Roman customs.